EFFECT OF SEVERE COLD WORK ON THE MICROSTRUCTURE AND SSC RESISTANCE OF SUPERDUPLEX STAINLESS STEEL DOWNHOLE TUBULARS

Authors: A J Leonard, BP Exploration Chertsey Rd Sunbury on Thames; J W Martin, BP Exploration Chertsey Rd Sunbury on Thames; A P Davis, BP America Inc.; M Ueda, Sumitomo Metal Industries Ltd. Pipe and Tube Company; S Kimura, Sumitomo Metal Industries, Ltd. Pipe and Tube Company

Source CORROSION 2010, March 14 - 18, 2010 , San Antonio, TX

Preview ABSTRACT

Cold worked superduplex stainless steels have been used as downhole tubulars for many years. However, problems during manufacture may result in areas of the tubulars exhibiting additional localized cold work. The effects of such damage on the SSC and SCC performance of tubulars has not been documented. To address these concerns, a program of work was conducted to characterize the microstructure developed in cold-worked superduplex stainless steel downhole tubulars as a result of damage during manufacture. Samples of material exhibiting different degrees of microstructural refinement as a result of the additional cold work were subjected to SSC testing. Test results and recommendations are presented.

INTRODUCTION

Cold worked duplex stainless steel downhole tubulars have been used successfully for many years in oil and gas production. However, there have been reported instances of failures attributed to chloride stress corrosion cracking. In one such instance [1,2], an unusual feature of the failure was that crack initiation had occurred within a region of “anomalous” microstructure (AM) associated with an area of surface grinding on the exterior surface of the tube. The microstructure in this initiation region was different from that expected in cold-worked superduplex stainless steel tubing: locally the material had been very heavily cold worked. The most likely cause of the microstructural disruption was galling caused by the drawing die during tube manufacture (final cold drawing), resulting in a longitudinal defect (score marks and plastic deformation) in the finished product. Generally, such defects introduced during cold drawing are removed by grinding.

Due to concerns that areas of anomalous microstructure may remain after surface grinding, a program of work was initiated to inspect tubulars for evidence of such heavily cold worked microstructure. This involved eddy current inspection, building on the work of [1] to identify tubulars containing additional cold work. In addition, laboratory corrosion testing was undertaken to assess the significance of such features to the integrity of tubulars if used in oil and gas production environments. This paper summarizes the findings of the corrosion testing element of these investigations.

MICROSTRUCTURE CHARACTERIZATION

Figure 1 shows the type of microstructure typical of superduplex downhole tubing, UNS S39274, 125 ksi grade. The typical hardness of this material is of the order of 400HV. Figures 2 to 4 show degrees of additional cold work introduced to the tubing and are categorised AM 1 to 3 respectively [2]. Hardnesses of these regions are typically greater than 475HV, with the microstructural disturbance extending from approximately 100μm to 250μm from the surface of the tube. For example, samples of AM2 microstructure, selected for corrosion testing, were defined as exhibiting a visual depth of 200μm before grinding and a maximum micro-hardness of 500-600HV. The hardness continued at above "background" (400HV) levels to a depth of 400μm.

(Figure in full paper)

For the purposes of a controlled corrosion testing program, it was not possible to extract multiple specimens from actual production pipe that all exhibited the same degree of cold work.